Remove hard coded language

By removing the hard coded language we enable a better experience with
the language switcher.

Changelog: changed

app/views/layouts/devise.html.haml

 - add_page_specific_style 'page_bundles/login'
 - custom_text = custom_sign_in_description
 !!! 5
-%html.html-devise-layout{ lang: "en" }
+%html.html-devise-layout{ lang: I18n.locale }
   = render "layouts/head", { startup_filename: 'signin' }
   %body.gl-h-full.login-page.navless{ class: "#{system_message_class} #{user_application_theme} #{client_class_list}", data: { page: body_data_page, qa_selector: 'login_page' } }
     = header_message

app/views/layouts/devise_empty.html.haml

 - add_page_specific_style 'page_bundles/login'
 !!! 5
-%html.html-devise-layout{ lang: "en" }
+%html.html-devise-layout{ lang: I18n.locale }
   = render "layouts/head"
   %body.gl-h-full.login-page.navless{ class: "#{system_message_class} #{user_application_theme} #{client_class_list}" }
     = header_message

app/views/layouts/signup_onboarding.html.haml

 - add_page_specific_style 'page_bundles/signup'
 - add_page_specific_style 'page_bundles/login'
 !!! 5
-%html.html-devise-layout{ lang: 'en' }
+%html.html-devise-layout{ lang: I18n.locale }
   = render "layouts/head"
   %body.signup-page.navless{ class: "#{system_message_class} #{user_application_theme} #{client_class_list}", data: { page: body_data_page, qa_selector: 'signup_page' } }
     = header_message

ee/spec/views/layouts/application.html.haml_spec.rb

@@ -13,6 +13,7 @@
   end
 
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 
   describe 'layouts/_user_notification_dot' do
     let(:track_selector) { '[data-track-action="render"][data-track-label="show_buy_ci_minutes_notification"]' }

spec/support/shared_examples/views/preferred_language.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'a layout which reflects the preferred language' do
+  context 'when changing the a preferred language' do
+    before do
+      Gitlab::I18n.locale = :es
+    end
+
+    after do
+      Gitlab::I18n.use_default_locale
+    end
+
+    it 'renders the correct `lang` attribute in the html element' do
+      render
+
+      expect(rendered).to have_css('html[lang=es]')
+    end
+  end
+end

spec/views/layouts/application.html.haml_spec.rb

@@ -11,6 +11,7 @@
   end
 
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 
   describe "visual review toolbar" do
     context "ENV['REVIEW_APPS_ENABLED'] is set to true" do

spec/views/layouts/devise.html.haml_spec.rb

@@ -4,6 +4,7 @@
 
 RSpec.describe 'layouts/devise', feature_category: :user_management do
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 
   describe 'logo' do
     it 'renders GitLab logo' do

spec/views/layouts/devise_empty.html.haml_spec.rb

@@ -4,4 +4,5 @@
 
 RSpec.describe 'layouts/devise_empty' do
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 end

spec/views/layouts/fullscreen.html.haml_spec.rb

@@ -35,6 +35,7 @@
   end
 
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 
   describe 'sidebar' do
     context 'when nav is set' do

spec/views/layouts/signup_onboarding.html.haml_spec.rb

@@ -4,4 +4,5 @@
 
 RSpec.describe 'layouts/signup_onboarding' do
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 end

spec/views/layouts/terms.html.haml_spec.rb

@@ -10,4 +10,5 @@
   end
 
   it_behaves_like 'a layout which reflects the application theme setting'
+  it_behaves_like 'a layout which reflects the preferred language'
 end